# IRF axes¶

Most IRFs are dependent on parameters, and the 1-dimensional parameter arrays are stored in columns. The following names are recommended:

• For energy grids, see here for basic recommendations. Column names should be ENERGY or ENERG_LO, ENERG_HI because that is used (consistently I think) for OGIP and Fermi-LAT. For separate HDUs, the extension names should be ENERGIES or EBOUNDS (used by Fermi-LAT consistently).
• Sky coordinates should be called RA, DEC, GLON, GLAT, ALT, AZ (see Coordinates)
• Field of view coordinates DETX, DETY or THETA, PHI for offset and position angle in the field of view (see Field of view).
• Offset wrt. the source position should be called RAD (this is what the OGIP PSF formats use).

In the specific case of point-like IRFs:

• The energy-dependent radius of the selected region of interest should be RAD_MAX

The IRF format specifications mention a recommended axis format and axis units. But tools should not depend on this and instead:

• Use the axis order specified by the CREF header keyword (see BINTABLE HDU)
• Use the axis unit specified by the CUNIT header keywords (see BINTABLE HDU)